Interpretive Summary: Arbuscular mycorrhizal [AM] fungi are naturally-occurring soil fungi that form a beneficial symbiosis with the roots of most crop plants. The plant receives benefits from the symbiosis which include increased: nutrient uptake from the soil, disease resistance, and drought resistance. These benefits make optimal usage of the symbiosis important for the sustainability of agriculture, especially so for organic farmers who cannot use chemical fertilizers or pesticides. One important way for farmers to better utilize these beneficial fungi is to inoculate their vegetable plants with them during the greenhouse growth phase, before the young plants are placed in the field. This way, the plants can take advantage of the symbiosis immediately. However, organic farmers have a challenge that conventional growers do not have when it comes to ensuring that the greenhouse inoculation phase is successful. The proper balance of potting soil nutrients is essential, notably sufficient nitrogen must be applied while limiting the level of phosphorus. Application of two high nitrogen and low phosphorus organic fertilizers were studied and compared to the application of a conventional chemical fertilizer for their effects upon the AM fungus colonization of roots of leek seedlings. We found that the application of blood meal tended to inhibit the colonization of roots by AM fungi, but fish meal produced seedlings that were well colonized. These results will help organic farmers ensure their investment in AM fungus inoculum is well utilized, and contribute to the economic and environmental sustainability of vegetable farms.

Technical Abstract:
The potential benefits of the arbuscular mycorrhizal [AM] symbiosis to crop growth and overall farm sustainability make effective utilization of AM fungus inoculum of particular interest to organic farmers. An important consideration in ensuring the colonization of vegetable seedlings by AM fungi during the greenhouse growth phase of production is supplying sufficient nitrogen and other nutrients for plant growth while limiting levels of phosphorus to avoid P-mediated inhibition of colonization. A greenhouse experiment was conducted to compare growth and AM fungus colonization of Allium porrum L cv Musselburgh seedlings in two organic potting media amended with high N and low P organic fertilizers vs a chemical fertilizer solution known to produce well-colonized seedlings. Though amendment of the media with the granular blood meal tended to produce the largest seedlings, formation of mycorrhizas was inhibited relative to 3X week-1 additions of inorganic control fertilizer or hydrolyzed fish (4.5% vs. 28% and 16% of root length, respectively). A follow-up experiment in a controlled environment chamber demonstrated no detrimental effect of 3X week-1 addition of blood meal in aqueous suspension relative to the chemical fertilizer. The incorporation of blood meal into the media in this experiment had no impact on mycorrhizal development through the first two weeks, but thereafter inhibited development of new infection units and inhibited colonization of roots relative to the other treatments after 5 weeks (7% vs. 38% of root length). Proper choice of organic fertility amendment is essential to produce AM fungus colonized seedlings ready to take advantage of the symbiosis immediately upon outplanting.